944 PHYSIOLOGY 



begins with the contraction of the auricle at 1 , which may or may not give 

 rise to a slight rise of pressure in the ventricles. As the auricular contrac- 

 tion dies away, the ventricular contraction begins at 2. This causes a very 

 rapid rise of pressure. Almost immediately after the beginning of the rise, 

 or sometimes synchronously, the auriculo-ventricular valves close at the 

 point marked 3. The pressure then rises rapidly in the ventricular cavity. 

 Directly it exceeds the pressure in the aorta, the aortic valves open at the 

 point marked 4, and the aortic pressure thereafter rises with the ventricular 

 pressure. During the whole duration of the ventricular contraction, the 

 aortic pressure remains somewhat below the ventricular pressure, showing 

 that blood is flowing continuously from ventricle into aorta. The rise of 

 pressure in the aorta may be at first rapid and then slow off gradually. 

 With the change of velocity of the rise of pressure, vibrations may be set up 

 at 5 especially in the aorta and also, but to a less extent, in the ventricle. 

 These vibrations are however often absent. At the end of the plateau 

 the pressure in the ventricle falls rapidly as this organ begins to relax. Its 

 pressure therefore falls below that of the aorta, and the aortic valves close, 

 the closure of the valves being followed by the so-called dicrotic elevation 

 or incisure at 7. The pressure in the ventricles then continues to fall, first 

 rapidly and then more slowly, until it reaches the line of zero pressure, and 

 remains at or near this line during the greater part of diastole. With a big 

 inflow there may be a slight rise towards the end of diastole, which may 

 be accentuated by the auricular contraction. If the chest is opened the 

 pressure in the ventricle never sinks below zero during any part of diastole. 

 The time relations of these events naturally varies with the frequency of 

 contraction of the heart. In a dog's heart beating about 100 times a minute, 

 the following phases in the ventricular tracing were determined by Hlirthle. 



(1) A small rise of pressure, due to contraction of the auricles, lasting 

 about -05 second. 



(2) A very steep ascent, rather above the middle of which the aortic 

 valves open. The beginning of the rise is generally marked by a sharp 

 secondary wave or by a sudden change in the direction of the curve. This 

 point is about -02 to -04 second after the beginning of the ventricular 

 contraction. 



(3) A prolonged stage, lasting about two-tenths of a second during 

 which the ventricle is contracting. During this time the tracing may show 

 a flattened top, the * plateau,' or a rounded summit. Near the beiiinniiiLi 

 of this plateau, a second wave may make its appearance if the rate, at which 

 the pressure rises, falls off suddenly. This second wave is therefore most 

 marked with a considerable output and a heart which beats forcibly and 

 rapidly. 



(4) A rapid fall, due to the relaxation of the ventricular muscle. Near 

 the upper part of this fall the aortic valves close; but it is generally difficult, 

 without comparison with the simultaneous aortic pressure curve, to make 

 out the exaci point of closure on the ventricular pressure curve. 



(5) A period lasting about two-tenths of a second during which the 



